Numerous methods have been used to monitor or track the direction of fixation of the eye. Infrared light emitters and sensors attached to glasses frames or to helmets have been used to detect changing infrared light patterns as the eye moves about (see, for example, U.S. Pat. Nos. 3,473,868 (Young et al.), 4,145,122 (Rinard et al.), 4,702,575 (Breglia), 4,735,498 (Udden et al.), 5,345,281 (Taboada et al.), and 5,382,989 (Uomori et al.)).
Video-based eye trackers and fixation monitors have been used to image the corneal light reflection against the background of the pupil of the eye. The position of the corneal light reflection within the image of the pupil yields an indication of the direction of fixation of the eye (see, for example, U.S. Pat. Nos. 3,462,604 (Mason), 4,836,670 (Hutchinson), 5,220,361 (Lehmer et al.), 5,327,191 (Shindo et al.), and 5,652,641 (Konishi)).
Other methods have been used to record the relative position of the first and fourth Purkinje images (the reflections from the anterior surface of the cornea and posterior surface of the crystalline lens, respectively). Their relative position is related to the direction of fixation of the eye (See, for example, U.S. Pat. Nos. 3,724,932 (Cornsweet et al.) and 4,729,652 (Effert)).
Electro-oculography has been used to record the direction of eye fixation by measuring weak electrical potentials on the skin that are related to eye position within the orbit (see, for example, U.S. Pat. No. 5,293,187 (Knapp et al.).
Search coils imbedded in scleral contact lenses have been used to record eye movements and eye position. The subject's entire head is positioned within a time-varying electromagnetic field. Depending on the angular position of the search coil within the electromagnetic field, more or less alternating current is induced in the coil as a measure of eye rotation (see, for example, Robinson D A, "A Method of Measuring Eye Movement Using a Scleral Search Coil in a Magnetic Field." IEEE Trans. Biomed. Electronics BME-10(4):137-145, 1963).
Each of the above techniques requires strict control of, or knowledge of, head position to determine where the eye is actually looking, that is, to determine the point of fixation of the eye. Apparatus must be attached to the head, or the head must be clamped within a head support, to provide accurate results. Furthermore, because these techniques monitor the position of the globe itself, and not the actual visual axis or point of fixation, the signal obtained must be calibrated against known directions of fixation of the eye, or known points of fixation, before useful measurements can be obtained.
A useful variation of eye tracking via the position of the corneal light reflection with respect to the pupil has been still photography of both eyes while the subject is instructed to look at a fixation light at or near the center of the camera lens (see, for example, U.S. Pat. Nos. 4,586,796 (Molteno) and 4,989,968 (Freedman)). With such a "photoscreening" device, asymmetry between the two eyes in the position of the comeal light reflection with respect to the pupils is indicative of misalignment of the eyes, that is, the clinical abnormality termed strabismus. Such photoscreening devices which image the pupils, however, have to be positioned a critical distance away from the subject to achieve proper focus of the pupils. It is often difficult to achieve and maintain this critical focus when photoscreening a freely-moving infant.
Apparatus for detecting the direction of eye fixation has been described in U.S. Pat. No. 5,331,149 (Spitzer and Jacobsen) wherein an array of illuminated pixels is presented to the eye in question, and light reflected from the fundus of the eye is detected by an array of photodetectors in registration with the array of pixels. As described therein, light is maximally reflected back toward the original pixel from which it came only when the fovea is aligned with that pixel, identifying fixation of the eye in the direction of that pixel. This apparatus fails to take into account the fact that substantial reflection of light occurs in a non-specular manner from the fundus, not only from the fovea but also from most areas of the fundus. Thus, substantial light will be reflected back toward every illuminated pixel, and the particular pixel aligned with the fovea may not be distinguishable by this technique.
Techniques which effectively track or monitor the optical projection of fundus landmarks out from the eye afford a somewhat more direct measurement of fixation direction. For example, the external location of the blind spot (the optical projection of the optic disc) can be monitored by test spots of light, presented within the presumed blind spot area, to which the subject responds if seen. A negative response indicates proper location of the blind spot. The subject must be alert and reliable, however, and accurate determination of the direction of eye fixation, or point of fixation, requires a calibration procedure as well as knowledge of the position of the eye in space.
A scanning laser ophthalmoscope can be used to lock onto, and track, the position of the optic disc, or the position of a branch point of a major blood vessel, in the fundus (see, for example, U.S. Pat. No. 4,856,891 (Pfibsen et al.). The visual axis itself cannot be tracked in this way, because there are no prominent landmarks in the fovea. Calibration with known directions of eye fixation, or known points of fixation, must still be performed, therefore, and precise alignment between the instrument and the pupil of the eye must be maintained at all times.
Although there are no prominent visible landmarks in the fovea of the eye, polarization effects can potentially be used to identify the fovea, utilizing the birefringent properties of the nerve fibers in the retina. The array of nerve fibers converging from all parts of the retina to the optic nerve head is characteristically unique. Many retinal nerve fibers diverge from the fovea and curve around to converge to the optic nerve head. Within the central four or five degrees of visual field, in the fovea, other nerve fibers, called Henle fibers, are arranged precisely radially, similar to the spokes of a wagon wheel. Both the retinal nerve fibers and the Henle fibers are known to have "form" birefringence, with the optic axis of the birefringence parallel to the direction of the fiber (see U.S. Pat. No. 5,303,709 (Dreher et al.) and klein Brink H B, van Blokland G J, "Birefringence of the Human Foveal Area Assessed In Vivo with Mueller-Matrix Ellipsometry," J. Opt. Soc. Amer. A 5:49-57, 1988)).
Further, a poorly-characterized source of dichroism exists in the human fovea, presumably related to lutein pigment particles that are aligned along the Henle fibers and along the ends of the retinal nerve fibers closest to the fovea. Both the birefringence of the nerve fibers, over the entire retina, and the dichroism in the area of the fovea can produce polarization-related changes in light that is reflected from the fundus of the eye. For polarization-related changes to be produced by the birefringence of the nerve fibers alone, the light must initially be polarized. The dichroism in the area of the fovea, however, produces polarization-related changes even in light that is initially non-polarized. The polarization-related changes that are produced at any one point are dependent on the direction and thickness of the nerve fibers at that point, as well as dependent in the foveal area upon the amount of dichroic pigment that may be present, aligned along the nerve fibers.
Scanning laser ophthalmoscopes have been described which yield maps of polarization-related changes across areas of the fundus of the eye (see Plesch A, Klingbeil U, Bille J. Digital laser scanning fundus camera. Appi. Opt. 26(8):1480-1486, 1987, and U.S. Pat. No. 5,177,511 (Feuerstein et al.)). The characteristic polarization-related changes which occur in the foveal area, however, have never been used to monitor or track the direction of fixation of the eye.
Techniques have been devised for actually measuring the amount of optical retardation produced by the retinal nerve fibers at points across the fundus of the eye (see, for example, U.S. Pat. No. 5,303,709). This measure of retardation is directly proportional to the thickness of the nerve fiber layer. Because the nerve fiber layer becomes attenuated and often irregular in serious eye disease such as glaucoma and optic nerve atrophy, determination of nerve fiber layer thickness provides a method for detecting such diseases.
Measurement of retardation produced by the retinal nerve fiber layer is hampered by a much larger amount of retardation that is normally produced by the naturally-occurring form birefringence of the cornea of the eye. Separate measurement of the corneal birefringence can be made, whereby the retardation produced by the corneal birefringence can be mathematically factored out of the total retardation to yield the retardation due to the birefringence of the nerve fibers. An alternative technique to avoid the complications introduced by the corneal birefringence is to compensate optically for the corneal birefringence as described in U.S. Pat. No. 5,303,709, whereupon the amount of retardation produced by the retinal nerve fibers can be measured directly. Such optical compensation of the corneal birefringence is technically demanding, however, involving special measurement and feedback systems.